1. Field of the Invention
The present invention relates to a liquid discharging apparatus having a head in which a plurality of liquid discharging portions, each having a nozzle, are arranged in a specific direction, and to a liquid discharging method using the head.
2. Description of the Related Art
Ink-jet printers are known as liquid discharging apparatuses. One type of ink-jet printer is a serial printer in which droplets are discharged from a head onto a recording medium while moving the head in the lateral direction of the recording medium, and in which the recording medium is moved in the feeding direction. Another type of ink-jet printer is a line printer having a line head extending along the overall width of the recording medium, in which only the recording medium is moved in a direction perpendicular to the lateral direction thereof, and in which droplets are discharged from the line head onto the recording medium (see, for example, Japanese Unexamined Patent Application Publication No. 2002-36522).
In print heads used in these ink-jet printers, when ink droplets are not discharged from any of the discharging portions for some reason, ink does not adhere to a position on a recording medium corresponding to the discharging portion, and a white stripe appears. This reduces the image quality. In some cases, ink droplets are discharged from a discharging portion in a direction deviating from the allowable range, or the amount of ink discharged from a discharging portion is quite small. These cases also reduce the image quality. In particular, since a line head includes more discharging portions than a serial head, a wider range of variations in ink discharging characteristics occur.
In a serial head, even when there are some variations in ink discharging characteristics among the discharging portions, the variations can be reduced by a method, called “superimposition”, for overlapping dots to bridge gaps between previously printed dots.
In contrast, since a line head does not move, it cannot perform overprinting on a prerecorded region. For this reason, variations among the discharging portions remain as in the direction in which the discharging portions are arranged, and result in conspicuous stripes.
Accordingly, in ink-jet printers, measures have been taken so that all the discharging portions of a print head properly discharge ink droplets. In particular, clogging of an ink discharging outlet, for example, due to drying of ink droplets is prevented by maintenance such as cleaning.
However, for example, in thermal ink-jet printers, problems that cannot be overcome by maintenance sometimes occur: for example, a heater for heating and discharging ink breaks, and an ink chamber malfunctions. In these cases, none of the discharging portions can discharge ink droplets. Since a print head having such a discharging portion cannot be repaired, it has been treated as defective.
For example, when it is assumed that the possibility of occurrence of such a defective discharging portion is approximately 1/40,000, one out of two hundred print heads, each having two hundred discharging portions, has a defective discharging portion. In this case, half of the print heads having multiple discharging portions, such as line heads, are defective, for example, when the recording paper is A4-sized and the resolution is 600 dpi because approximately five thousand discharging portions are prepared for one color, that is, approximately twenty thousand discharging portions are prepared for four colors. Therefore, the production yields of print heads are significantly reduced.